2 Corinthians 3.18 is an important verse in the realm of transformation of lives. In it, the Apostle Paul makes sure we understand that what we behold is what we become. As we see God, we are transformed by God to be like God.
When we talk about "seeing" God we're not talking about with the physical eyes but what A.W. Tower calls the "gaze of the soul." We behold God when we gaze into the Word prayerfully.
But the other side of this is we can hurt ourselves by what our gaze. What we behold is what we become. Psalm 115 supports this: those who worship them [idols] will become like them.
It doesn't take very much mental power to think about what you can look at, what you can behold that will make you become empty, vain, selfish, ruthless, darkened, immoral, impure, or any other trait associate with the Kingdom of Darkness.
So remember what Jesus said: the eye is the lamp of the body. If it's good, light goes to the whole body. If the lamp is dark, just how dark will the darkness be?
You become what you behold.